An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.
NGC 604, a giant H II region in the Triangulum Galaxy
Dark star-forming regions within the Eagle Nebula commonly referred to as the Pillars of Creation
Orion Nebula
A small portion of the Tarantula Nebula, a giant H II region in the Large Magellanic Cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
Astronomer Henk van de Hulst first theorized hydrogen could be traceable in interstellar space using radio signals.
Plaque commemorating the discovery of 21-cm radiation from the Milky Way
Left to right: Jan Oort, Hendrik C. van de Hulst, Pieter Oosterhoff. Jan Oort had a pivotal role in the research that lead to the discovery of molecular clouds.
Penzias and Wilson with the Holmdel horn antenna used to detect microwave emissions from the Big Bang